( 664 ) 



the corona. This beam mav, accordingly, arrive witii a quite dilfereiit, 

 say a greatei-, (livergence and consequently display a smaller intensity 

 in the spectrum, than the neighbouring beam, first considered. 



Approaching still nearer to tiie absorption-line we shall come across 

 waves that I'each the Earth in beams whose divergence is smaller 

 again, showing increased intensity, etc. It is plain that in this ^vay 

 periodical alternations of" light and dark on eithei- side the central 

 absorption line must arise. The waves, corresponding to the middle 

 of one of" these fringes, will have achieved exactly one whole bend 

 ^i.e. the distance between two consecutive i)oints of inllexioii of the 

 l)ath) more, or less, than tlutse corresponding to the middle of the 

 adjacent fringes. 



From the familiar type of the dispei\sion curve it follows directly, 

 that, in moving away from the absorj)tion line, to e(iual ditlerenccs 

 iji refractioji constant increasin<>- differences in wavelejmth will 

 answer. The distance between the f"i-inges will accordingly inci-ease 

 from the centre to either side, as has in fact been observed. 



Our explanation requires besides, that this system of faint lijies 

 should be visible only when suidight reaches us exactly along a 

 coronal streamer of sufficient length. In my last pa])er (I.e.) I showed 

 that, in case this condition is fulfilled, the average shading of ihe 

 Frauidiofer lines must be abnormally weak. It is therefore not to be 

 wondered at, that on the plate, [jlaiidy displaying the peculiar structui-e 

 of H ami K, the shading really was unusually faint. IJut the 

 foruiation of a well defined line-system demands a further condition 

 to be fulfilled, \iz. that the configuration of that part of the (rotating) 

 corona wg are just looking through, offers all but the same asi)ect 

 as long as the photographic [)late is exposed. This, of course, requiring 

 very special circumstances, we see why even in cases, in which the 

 shading of the Frauidiofer lines is weak, the fringes may be missing 

 all the same. 



In a few cases has a like structure been observed with some 

 strongly widejied emission lines of the arc-spectrum. Kayskr came 

 across this phenomenon in a line of the lead-spectrum ^) ; Rowi.anü 

 too seems to have observed it once ; and after many vain endeavours 

 Jr.wKLL succeeded in obtaining a [)hotograj)li of the arc spectrum of 

 calcium, in which at H and K the series appeared rather distinctly. 

 This })late was obtained by using an extremely powerful direct current 

 and exposing for three or four seconds only. Under these conditicuis 



^) H. Kayser, Handltucli dor Spectroscopie, II, p. 353. 



